Asthma is a clinical syndrome characterized by reversible airway obstruction, airway inflammation and hyperresponsiveness to a wide variety of stimuli. However, the pathogenesis has remained poorly defined. Certain microorganisms have been implicated in the exacerbation of asthma, particularly viruses, chlamydia and Mycoplasma pneumoniae. Several studies have shown that infection with M. pneumoniae results in increased bronchial hyperresponsiveness and airflow obstruction in non-asthmatic patients that can last several years after infection. In addition, an animal model of airway disease similar to asthma can be produced with Mycoplasma pulmonis. We hypothesized that patients with chronic, stable asthma would exhibit evidence of mycoplasma in the upper and lower airways. Furthermore, treatment with clarithromycin, a macrolide with known activity against mycoplasma species, would result in improvement in their asthma.